毘沙門天 (pronounced as 'Bishamonten' in Japanese, 'Vaiśravaṇa' in Sanskrit) is one of the most popular 'tenbu' (天部, deva) in Japan. Devas are former gods of Hindu mythology which became the guardian deities of Buddhism.
He has various characteristics and figures in different regions. In Japan, he is a warrior god wearing Chinese-style armor, holding a spear and a miniature pagoda (bishamonten's pagoda). In Chinese folklore, he has an umbrella and a silver mouse.
He is a member of the Four Heavenly Kings (Kanji: 四天王) and the guardian god of the northern direction. Another name for him is 多聞天 (Tamonten, lit. 'Much hearing god').
In Japanese folklore, he is a member of 'Shichi Fukujin' (七福神, the Seven Lucky Gods).
Toramaru Shou in the Touhou series is a disciple of Bishamonten, as is Uesugi Kenshin in the Sengoku Musou series.